The present invention relates to focal plane array technology, and more specifically, to a readout cell for use in hybrid and monolithic focal plane arrays.
In a hybrid focal plane array, arrays of photodetectors are coupled to or are part of an integrated circuit that includes integrating readout circuits and an output multiplexer. These circuits may be a hybrid focal plane array in which the detectors and the readout circuit are on separate substrates that are mechanically and electrically connected or monolithic focal plane arrays where the detectors and readout circuitry are fabricated in a common substrate.
A conventional hybrid staring infrared focal plane array typically employs an array or matrix of infrared detectors, made of some suitable material such as mercury cadmium telluride or indium antimonide, etc., attached by indium bumps (one per detector) to a readout array. The readout array integrates a photocurrent from each detector for a specified period of time, then multiplexes the signals from the detectors to a single video output (or a small number of outputs). Interface circuitry which sets detector bias voltage, integrates the photocurrent, and multiplexes the output signal, is contained in a repeated unit cell with the same dimensions as the detector spacing. The present state of the art for midwave and longwave hybrid infrared staring focal plane array, is a 128.times.128 array of detectors on 40 micron centers, and the trend of technology development is to larger arrays of smaller detectors. With a cell size of 40 .mu.m or smaller, the limit to the signal to noise ratio is typically the shot noise in the number of photoelectrons that can be integrated into a capacitor in the unit cell. The limiting signal to noise ratio varies as the square root of the charge that can be integrated. A conventional direct injection circuit uses an integrating capacitor that is terminated in a fixed voltage. The maximum charge that can be integrated is the product of the capacitance and the voltage limit of the process. The shot noise on this charge sets the limit to the dynamic range of the focal plane array.
One class of circuit approaches that attempt to improve dynamic range include various gate modulation schemes. These circuits generally have problems with noise from sources other than shot noise, to the point that there is little or no real gain in dynamic range. They also may have problems with high sensitivity to fabrication process parameters, so that the circuit yield is likely to be low, and the circuits suffer from nonuniformity, and nonlinearity that makes nonuniformity correction difficult. Other known circuit approaches require high value resistors that are not normally available in commercial complementary metal oxide semiconductor (CMOS) processes and are difficult to control, so that yield is at risk and uniformity of response is degraded.
In some applications, an infrared focal plane array is required to have a quite large instantaneous dynamic range. A case of particular interest is an infrared focal plane array that is subjected to a high level of background infrared radiation from a hot, imperfectly transmitting window in front of the array, as, for example, in an infrared guided missile in which the window is subject to aerodynamic heating.
Reference is made to an article entitled "Readout mechanisms for infrared focal plane arrays," by A. F. Milton, published in the Proceedings of the SPIE, Volume 443, pages 110-119, Aug. 25-26, 1983. In this article, at page 116, and with reference to FIG. 10 on page 119, it is mentioned that ramping has been proposed as a technique to increase N.sub.omax in an IR MIS array. It is also mentioned that "Ramping will however not work if the background is variable. If the background heats up the array will saturate and if it cools off tunneling will occur."
Accordingly, it is an objective of the present invention to provide a readout arrangement for a focal plane array that is adapted to integrate a large amount of charge. Another objective of the invention is the provision of a readout arrangement for a focal plane array that has an improved signal-to-noise ratio. Yet another objective of the invention is the provision of a readout cell for a focal plane array that suppresses background signal due to a heated infrared dome. A still further objective of the present invention is to provide an improved readout cell for a focal plane array that operates over a large dynamic range. Another objective is to provide a readout design that does not require any unusual or unique processing steps, so that is can be fabricated with commercially available MOS or CMOS integrated circuit processes